An active matrix substrate usable for a liquid crystal display device or the like includes a switching element such as a thin film transistor (hereinafter, referred to as a “TFT”) in each of pixels. As such a switching element, a TFT including an amorphous silicon film as an active layer (hereinafter, referred to as a “amorphous silicon TFT”) or a TFT including a polycrystalline silicon film as an active layer (hereinafter, referred to as a “polycrystalline silicon TFT”) is in wide use.
It has been attempted to use a material other than amorphous silicon or polycrystalline silicon as a material of an active layer of a TFT. For example, Patent Document 1 describes a liquid crystal display device including an oxide semiconductor film formed of InGaZnO (oxide containing indium, gallium and zinc) as an active layer of a TFT. Such a TFT is referred to as an “oxide semiconductor TFT”.
An oxide semiconductor TFT is operable at a higher speed than an amorphous silicon TFT. An oxide semiconductor film is formed by a simpler process than a polycrystalline silicon film and therefore is applicable to a device requiring a large area size. For these reasons, use of an oxide semiconductor TFT for a display device or the like as a high performance active element producible at lower cost is now spreading.
Recently, a mobile device such as a smartphone, a tablet terminal or the like includes a compact high-definition liquid crystal display device. It is preferable to increase the pixel aperture ratio in order to realize bright display and extend the driving time period. The “pixel aperture ratio” refers to a ratio of size of an area usable for display (size of the area excluding non-display region including lines, TFT and the like) with respect to the area size of one pixel region. As the pixel aperture ratio is higher, the utilization factor of light from a backlight unit is higher. Such an increase in the pixel aperture ratio realizes bright display and decreases power consumption.
However, it is not easy to increase the pixel aperture ratio in a compact high-definition display device usable in a smartphone or the like because of the restriction on the minimum width of lines (process rule) or the like. Various technologies have been proposed in order to increase the pixel aperture ratio.
The pixel aperture ratio may be increased by use of, for example, the above-described oxide semiconductor TFT. A reason for this is the oxide semiconductor has a high mobility, and therefore, even a smaller oxide semiconductor TFT provides a level of performance higher than, or equal to, that of an amorphous silicon TFT. Usually, an area where a TFT is provided is blocked against light by a light blocking layer (e.g., by a gate electrode or a black matrix) because such an area is to be prevented from being irradiated with light or because such an area is difficult to be used for display. A decrease in the size of TFT increases the pixel aperture ratio by such a decrease.
It is also conceivable to locate a gate electrode and source and drain electrodes close to each other in order to increase the aperture ratio. For example, Patent Document 2 describes a structure including a TFT provided on a gate line to decrease the non-display region. A compact TFT realized in such a manner increases the size of the area usable for display with respect to the size of the pixel region, which increases the aperture ratio.